1. Field of the Invention
The present invention relates to a radar apparatus utilized as a vehicle radar system or the like, and in particular relates to a radar apparatus which can detect a deterioration in sensitivity of the radar apparatus due to an abnormality of the radar apparatus itself or to snow or mud attached to a radome on the front of the radar apparatus.
2. Description of the Related Art
A vehicle radar apparatus has heretofore been made practicable for obstacle detection for preventing collision with surrounding obstacles at the time of travelling along narrow roads, or when putting a vehicle into a garage. Moreover, implementation of a radar apparatus which detects a target object at a relatively short distance, a high speed and at high accuracy, has been recently promoted as an warning system for preventing a rear-end collision with a vehicle driving ahead at the time of driving a vehicle, or as a detection device for a cruise control device (ACC) which maintains a constant vehicle distance with a preceding vehicle while travelling.
For example, in the publication of Japanese Patent No. 2567322, is disclosed a multi-beam radar apparatus in which detection accuracy is improved by radiating spatially overlapping radar beams of millimeter-wave band, using a plurality of transmitter-receiver devices, and changing the combination of the transmitter-receiver circuits. Moreover, in the publication of Japanese Patent Application, First Publication No. Hei. 10-145129, is disclosed a scan beam radar apparatus which oscillates and scans a radio beam radiated from a primary radiator by means of a rotating reflective body and converges the reflected beam by a dielectric lens to thereby radiate the reflected beam in the vehicle driving direction.
However, with such a radar apparatus, for example with the multi-beam radar apparatus, the position of the target object (the azimuth and the distance with respect to the vehicle) is calculated by combining received data observed for each combination of transmit and receive signal circuits. Therefore when a fault occurs in any one of the plurality of transmit and receive circuits, or when snow or mud is unevenly attached to the radome in front of the radar apparatus, an error occurs in the calculated position of the target object. Moreover with the scan beam radar apparatus, in the case of uneven attachment of snow or the like, a similar detection error occurs. Furthermore, when snow or mud is adhered over the whole surface of the radome, then with two radar apparatus where azimuth error is unlikely, the detection sensitivity drops uniformly over the whole azimuth. Therefore a phenomena arises where the measurement range in which the target object can be detected is shortened.
Moreover, in the case where an error occurs in the detection position of the target object due to this phenomena, then for example, with the beforementioned cruise control unit there is a problem in that smoothness is impaired.
Therefore, it has heretofore been necessary to regularly measure the sensitivity of the radar apparatus and can that here is no abnormality therein, such as a sensitivity deterioration in the respective transmitting and receiving circuits, and this sensitivity measurement is performed, for example at regular checks. Moreover, since there is the possibility of the occurrence of a position detection error attributable to a deterioration in sensitivity during travelling or to uneven attachment of snow etc. to the radome, there is a demand for some means which enables sensitivity measurement during travelling. There has been proposed a method for judging the condition of the radar apparatus from, for example the signal strength of a reflection signal from a preceding vehicle, or the signal strength of a reflection signal from a fixed object on the road shoulder such as a guard rail.
However, heretofore sensitivity measurement for radar apparatus has been performed by transporting a vehicle mounted with the radar apparatus to a test environment where a reference target is installed, transmitting and receiving a radar beam to/from the reference target with the vehicle parked at a predetermined test position, and measuring the reception level. Hence there is a problem in that much time and man-hours are required. Moreover, with this method, there is a problem in that in the case where a partial fault occurs in the constituent circuits between periodic check, or in the case where due to uneven snow attachment etc. there is a temporary drop in sensitivity, an error in the target object position cannot be avoided.
Furthermore, with the method where judgment is made using the signal strength of the reflection signal from the preceding vehicle or an object on the road shoulder, with the preceding vehicle or with the object on the road shoulder, there is not always the situation where these have the same reflectivity, and with a travelling environment which also includes weather conditions, the signal strength of the detected reflection signals can be completely different. Consequently, in the case where a drop in the signal strength, for example of the receive signal is observed, it is difficult to make a discriminating judgment as to whether this is a deterioration in the transmit and receive signal strength of the radar apparatus, or that the reflection signal from an object with a low reflection level is being normally received, or simply that there is merely a change in the weather conditions. Hence, a radar apparatus has been desired which can measure the sensitivity while travelling, and diagnose deterioration without being influenced by these individual environmental conditions.
The present invention has been completed in consideration of the above problems, and disadvantages with the object of providing a radar apparatus which detects and judges a drop in sensitivity of the radar apparatus in the normal travelling condition, without transporting the vehicle to a special environment, so that problems based on errors in the target object position can be avoided beforehand.
To solve the above described problems, with the present invention, with a radar apparatus used mounted on a vehicle and having: a beam transmission device (for example, the transmission section 30 in the embodiment) for rating a radar beam as a transmission signal; a bean reception device (for example, the reception section 40 in the embodiment) for receiving a reception signal reflected from a target object which is within a radiation range of the radiated radar beam; and a processing unit (for example, the detection/control section 50 in the embodiment) for detecting the position of a target object from the transmission signal and the reception signal, the processing unit includes a preceding vehicle judgment device (for example, the preceding vehicle judgment circuit 51a in the embodiment) for judging if a preceding vehicle travelling in front of the vehicle (the subject vehicle) in approximately the same direction as the subject vehicle is the same vehicle, as previously detected and a signal strength comparing device (for example, the signal strength comparing circuit 51c in the embodiment) for comparing the signal strength of a reception signal reflected from the preceding vehicle, with a signal strength of a reception signal reflected from the preceding vehicle a predetermined comparison reference time prior, to thereby calculate a change amount in signal strength. When the change amount calculated by the signal strength comparing device drops beyond a previously set threshold value and it is judged by the preceding vehicle judgment device that the preceding vehicle is the same vehicle, the processing unit judges that the detection sensitivity of the radar apparatus has dropped.
With the above described construction, the same preceding vehicle ahead of the sect vehicle is made the target, and the signal strength of the current reception signal reflected from this preceding vehicle, and a signs strength of a reception signal detected at a predetermined comparison reference time prior are subjected to a comparison calculation, and when the change amount of the signal strength drops beyond a previously set threshold value, it is judged that the detection sensitivity of the radar apparatus has dropped. Consequently, it is not necessary to transport and set up the vehicle in a special measurement environment, and carry out sensitivity measurements. Moreover, abnormality or a drop in sensitivity of the radar apparatus during travelling can be diagnosed without depending on the travelling road environment or the like. Here, the comparison reference time may be set to a fixed time within a range of several seconds to several minutes. However this may also be set for example in map form corresponding to vehicle speed, or as mentioned after, may change corresponding to relative distance change amount between vehicles.
Preferably the radar apparatus is constructed with the processing unit further incorporating a distance change calculating device (for example, the distance change calculating circuit 51b in the embodiment) for calculating a change amount of relative distance between the preceding vehicle and the subject vehicle, and comparison of the reception signal is made when the calculated change amount of the relative distance is below a previously set change amount.
In the case where the preceding vehicle is the same vehicle and the subject vehicle travels so as to pursue the preceding vehicle, the relative distance between the preceding vehicle and the subject vehicle in general always changes little by little with time. Therefore, when the relative distance between vehicles changes before or after elapse of the comparison reference time, the reception signal strength changes corresponding to that distance change. However, with the above construction, since the strength comparison of the reception signals is made when the change amount of the relative distance between vehicles is less than a set change amount, erroneous judgment accompanying the relative distance change can be reduced, and an effect of improving the accuracy in detecting sensitivity deterioration can be obtained.
Preferably the radar apparatus is constructed as mentioned above with the processing unit further incorporating a distance change calculating device for calculating a change amount of relative distance between the preceding vehicle and the subject vehicle, and the comparison reference time is changed base on to the calculated change amount of the relative distance.
With the above construction, the comparison reference time is changed corresponding to the change amount (for example the change amount between two measuring points or the change amount within a constant time) of the relative distance between the preceding vehicle and the subject vehicle. For example, in the case where the relative distance between the preceding vehicle and the subject vehicle increases, the comparative reference time shortens corresponding to that distance change amount, while when the relative distance is constant at separated locations, the comparative reference time lengthens, or the time returns to the original time. As a result, a relative distance difference between two points for relative computation of the reception strength can be suppressed to below a fixed range, and hence erroneous judgement due to the relative distance becoming different can be reduced, and provided the preceding vehicle is the same, deterioration judgment can be continuously performed.